In electron-beam projection microlithography as used in the fabrication of semiconductor integrated circuits, a circuit pattern defined by a reticle or mask is irradiated with an electron beam emitted from an electron gun. Variable-shaping and cell-projection systems have been used as electron-beam delineation apparatus. Lanthanum hexaboride (LaB.sub.6) is conventionally used in electron-gun cathodes that are operated in a relatively low-temperature environment, i.e., the operation temperature of a LaB.sub.6 cathode is about 1000.degree. C. to about 1600.degree. C. These temperatures are significantly lower than the operating temperatures of conventional cathodes, such as tantalum (1900.degree. C.) or tungsten (3000.degree. C.), so that the electron gun components are in a low-temperature environment). LaB.sub.6 electron guns typically possess a brightness of at least 10.sup.5 A/cm.sup.2.multidot. sr and provide an emittance of several tens of .mu.m.multidot.mrad (at an acceleration voltage of 30 kV).
Conventional variable-shaping and cell-projection electron-beam delineation apparatus cannot provide the high resolution and integration densities necessary to produce the semiconductor integrated circuits demanded in recent years. Consequently, there is a current need for electron-beam projection apparatus that provide both high-resolution and high-throughput electron exposure of the substrate.
There has been recent interest in projection apparatus that perform pattern-image transfer using a mask or reticle having a relatively large optical field representing one die that is divided into field segments ("mask subfields") to be individually projected onto the substrate surface. The die pattern image is typically transferred using a "step-and-repeat" transfer scheme in which individual mask subfields are sequentially transferred to corresponding "transfer subfields" on the substrate. The transfer subfields are produced on the substrate surface in locations relative to each other such that the transfer subfields are "stitched" together in the correct order and alignment to reproduce the entire die pattern on the substrate surface (as described, e.g., in U.S. Pat. No. 5,260,151, incorporated herein by reference).
Having to incrementally transfer multiple subfields per die requires more time than transferring the entire die pattern in a single exposure; thus, the former can have a slower throughput than the latter. In order to increase throughput in the subfield-by-subfield step-and-repeat scheme, increasing the size of the mask subfields transferred per exposure (and thus reducing the number of mask subfields per die) has been considered. For step-and-repeat lithography, the required width of a single mask subfield can be several hundreds of square micrometers or more. Such a mask subfield is many times larger than the mask subfields used with conventional electron-beam projection delineation apparatus. Larger mask subfields require an electron gun capable of producing an irradiating electron beam having a uniform intensity distribution over a wider field than is obtainable with currently available equipment.
Two key parameters of an electron gun are its emittance and brightness. For step-and-repeat lithography, an electron gun can be used having low brightness and high emittance. However, it is difficult to decrease the brightness and/or increase the emittance of an electron gun that operates in a space-charge limited condition or a temperature-limited condition (i.e., a low-temperature electron-gun environment). An electron gun used for step-and-repeat lithography must provide a brightness of about 10.sup.3 A/cm.sup.2.multidot. sr or less and an emittance of several thousands of .mu.m.multidot.mrad at an acceleration voltage of 100 kV or greater. These specifications are about 1/100 of the brightness and about 100.times. the emittance provided by conventional electron guns.
Additionally, electron guns that operate under a temperature-limited condition typically experience fluctuations in the emission current due to fluctuations in the cathode temperature. Fluctuations in the emission current cause variations in the electron dosage to which a substrate surface is exposed. Excessive fluctuations in the electron dosage delivered to a substrate surface (such as the electron dosage delivered to a layer of resist on a wafer substrate) result in imprecise transfer of pattern images.
Accordingly, there is a need to minimize fluctuations in electron dosages delivered to the substrate surface in order to perform high-precision pattern-image transfers using electron guns operated under temperature-limited conditions. Conventional electron guns provide electron dosages having a stability level of about 3% per hour. A stability of 0.5% or less per hour is desirable.
Additionally, there is a need for electron-beam projection-microlithography apparatus that generate electron fluxes at high emittance and low brightness while providing high throughput with uniform electron-flux irradiation intensity of the reticle or mask and, ultimately, of the substrate surface.